the honest position is that hobby-level interpretation of a chromatogram is worth something but not much
#hplc-massspec 2025-10-30
Thursday49 messages11 participantstimes are UTC
Highlights from this day
- LC_MS_Lena — a shoulder on a peak is information, and usually bad information 20:00
- VialBot — On this day 3 years ago this channel logged 19 messages. 20:07
- oasis_ola — for the archive deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant 20:15
- VialBot — New independent result logged — TFC, lot A-2601, purity 98.1% (VendorInvestigate). 20:38
- oasis_ola — whats the void volume peak, is that always solvent 21:06
shallow gradient
nobody runs enough blanks, including me, from memory
integration choices move the number by a few tenths easily. that is most of your inter-lab variance, i could be wrong
retention time is a hint, not a fingerprint. two different peptides can co-elute perfectly happily
retention time is a hint
214nm sees the amide backbone so it sees everything. 280 only sees aromatics
run a blank
two labs a point or two apart on the same lot is normal and gets mistaken for one of them being wrong constantly
thats a charge state
why does integration change the number so much
whats a plus one dalton shift usually
deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant
unrelated but i have had a lot where the mass was exactly right and the purity was 96.8. wrong question, right answer
if the lab reports area percent without stating the wavelength, the number is less useful than it looks
i asked VendorInvestigate for the raw trace and they sent it. that is the right answer to that question
i was wrong about tailing factor. it is usually the injection or the pH, not the column
oxidation is about plus 15.99 Da and is usually methionine or tryptophan
a shoulder on a peak is information, and usually bad information
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coming back to this plus sixteen, is that always oxidation
has anyone ever had a result where the mass was right and the purity was wrong
[edited]On this day 3 years ago this channel logged 19 messages.
for the archive deamidation is about plus 0.98 Da. it is not a rounding error, it is a degradant
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monoisotopic and average diverge more the bigger the molecule. say which one you are quoting
thats co-elution
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thats the solvent peak
can you tell a dimer from the chromatogram alone
a dimer usually shows up late and at roughly double the mass. the chromatogram alone will not tell you
New independent result logged — TFC, lot A-2601, purity 98.1% (VendorInvestigate).
how do you spot a deletion in a sequence from mass alone
quick one is there any point running a blank if the sample is the question
[edited]within inter-lab range
whats a normal gradient for a peptide this size
baseline is drifting badly, is that the column or the mobile phase
whats the void volume peak, is that always solvent
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does retention time on its own tell you what the compound is
*Medutest not the other one
integration difference
show me the spectrum
baseline drift on a gradient is normal. baseline drift on an isocratic hold is your column
how would a truncation sequence show up
a certificate with a number and no chromatogram is asking you to trust the integration you cannot see
ms/ms or it isnt identity
a warm transit can put a visible degradant peak on a chromatogram. that is what 6 days at ambient does
relative retention time against a standard is comparable between labs. absolute minutes are not
coming back after 26 months, what changed in how you read these
if the number you are staring at is a third of what you expected, do the charge state arithmetic before you panic
do the arithmetic
plus 16 is oxidation